well I have Aspergers Syndrome, and there is currently some dispute as to weather I might have some form of PDD so all things being equal I know quite a lot about ASD's and Aspergers Syndrome, however I am no expert, what type of ASD does your son have if it is CDD (Childhood Disintegration Disorder) then what your describing is normal as children with CDD begin to lose skills between the ages of 2 and 10 and yes that includes bowel and bladder control. There are two things that make me suspect CDD, first he is eight and second he only recently became bowel incontinent. An eight year old does not just start pooping in their pants for no reason.
However if his poops are lose and runny like dioreaha I would say it is Probably not CDD unless there has been a confirmed diagnosis of it, and probably due to stress or a bowel problem either constipation (Encorperises or an intestinal infection) in either case you should get him down to the gp, you will know if he is constipated because the the hard poops block up the gut, duadeum,and rectum,this causes stretching of the anal sphincters wich allows ssoft and watery poops to pass with out the child knowing and eventually causes sensory loss in the area as well. If you suspect a intestinal infection ask yourself the following question has he been sick or is he passing large amounts of dioreaha is he feeling tired a lot or does he seem dehydrated or weak in anyway, if you suspect it may be related to stress you should again see your gp to get an appointment to see a counselor that has had experience dealing with children with ASD's .
One thing I would suggest rather then putting him in nappies (unless CDD is confirmed) is stuffing the back of his underwear with first a few sheets of kitchen roll paper then a whole load of toilet paper, it will keep his underwear clean and make clean up easier, as for the smell you could try chlorophyll tablets, though they take around 2 or 3 weeks to work.
("NOTE" The kitchen roll paper / toilet paper stuffed under doesn't work for watery poop you may have to use a nappy of some sort if his poops are watery)
It depends on the water content of the poop as to how badly it will stain underwear I have a fear of he potty monster from when I was much younger, I fell in the potty while it was eating the poopies and it tried to eat me after that i began holding my poops and developed encorporises this made me incontinent of poop, I eventually opted to wear nappies, but enitioly i used the kitchen roll paper / toilet paper stuffed underwear method when I was in school, that being said I would avoid using nappies on him unless it is absolutely nesisery, As an Adult Baby, (not related to Pedophiles), I happen to know their is a strong link between people with ASD's / Aspergers and Infantilism, it has been proven that people with an ASD or Aspergers are 17 times more likely to develop some form of infantilism and I would hate to see any child become an infantilist.
What triggers his incontinence?